1. Field of the Invention
The present invention relates to the field of positively acting differentials for motor vehicles.
2. Prior Art
Land vehicles, such as automobiles, trucks, buses and the like, typically utilize what has become known as an "open differential" for the final drive system. In such a differential, bevel gears are coupled to the inner ends of left and right collinear axles. These bevel gears engage accompanying bevel gears mounted on a pinion pin in a differential case. The differential case, in turn, has a ring gear thereon, with the ring gear and differential case being driven in rotation about the axis of the axles by a pinion gear on the drive shaft. The angular velocity of the ring gear and differential case determines the average angular velocity of the two axles. However, the bevel gearset within the differential case between the two axles allows one axle to turn faster and the other axle to turn slower than the ring gear and differential case at any particular time. This, of course, is highly desirable in normal driving, as it allows the axle coupled to the outer wheel to rotate faster than the axle coupled to the inner wheel when going around a curve or turning a sharp corner. It also causes the drive system to deliver the same drive torque to each of the two axles to avoid a tendency for the vehicle to pull to one side or the other when power is applied or removed. The amount of torque that can be transmitted through an open differential is limited to that able to be carried by the wheel with the least amount of traction.
There are certain situations, however, where the aforementioned characteristics of an open differential become undesirable. In particular, when one wheel loses traction, the torque which will be delivered to the wheel with traction will be no higher than the torque delivered to the wheel without traction. For instance, with one drive wheel on ice and the other drive wheel on dry concrete, the torque delivered to the drive wheel on dry concrete will be no higher than can be carried by the wheel on ice. A locking differential, however, will effectively couple the two axles together so that they turn in unison, forcing rotation of the drive wheel with greater traction along with the rotation of the wheel with lesser traction. The locking differential, as opposed to the open differential, can transmit as much torque as can be carried by the wheel with the most traction. High performance vehicles, off-road vehicles and the like may similarly take advantage of the characteristics of locking differentials to improve their traction performance.
The present invention is a significant improvement in a prior art differential manufactured by Vehicular Technologies, Inc., assignee of the present invention, and sold under the trademark "Performance." That prior art device is shown in the exploded view of FIG. 1. Referring to FIG. 1, the splined inner end of axle 20 engages mating splines in a coupler 22, with a similar coupler 22 at the opposite side of the assembly similarly mating at the inner end of the other axle, not shown. The axle bears against a pinion pin 34 that provides the inward limit for the axial position of the axle. In some embodiments (not shown), shims or thrust washers are used between the pin 34 and the ends of the axles 20 to establish the desired inward limit. A locking differential reuses some components of the open differential supplied with the vehicle. In particular, the pinion pin 34 that carries the bevel gears of the open differential may be reused although the bevel gears are not used. For this reason, pin 34 is referred to as a pinion pin even though it does not carry gears in a locking differential of the type shown. In the version shown, the axles are retained in position by C clips 24.
The couplers 22 have a plurality of teeth 26 on the face thereof which may mate with corresponding teeth on the faces 28 of drivers 30, depending upon the axial position of the drivers. The drivers 30, in turn, have saddle-shaped depressions 32 on the opposite faces thereof for loosely surrounding the pinion pin 34 driven by the differential case 54 (see FIG. 2 for the position of the pinion pin in the overall differential assembly). The drivers 30 each have springs 36 in angled blind holes in the driver, the springs acting on pin 34 to both elastically encourage the drivers to a position having the pin 34 aligned with the center of the saddle-shaped depressions, and to elastically encourage the drivers axially outward away from the pin 34 into engagement with the couplers. Spacers 38, together with any shims or thrust washers that may be required (not shown), establish and retain the couplers 22 at the desired separation from pin 34. Finally, pins 40 on the drivers 30 fit within slots 42 on the opposing face of the opposite driver and function to control the angular displacement of the drivers to each other.
In the final assembly, the springs 36 encourage the toothed face of the drivers 30 into engagement with the toothed face of couplers 22, and there is sufficient clearance between the saddle-shaped depressions 32 and pin 34 in the final assembly for either driver to move toward the pin 34 sufficiently to allow the teeth of a driver 30 to ride over the teeth of the associated coupler 22.
The operation of the prior art device may be explained as follows. With the teeth of the corresponding driver and coupler pairs engaged, the differential housing may rotate, carrying pin 34 from contact with one side of the saddle to the other, a displacement of (depending on the size of the design) 4 to 7 degrees. This free travel, or backlash, is essential for correct positioning of the differential components during the transition from driving to coasting and vice versa. The drivers are retained with respect to each other by pins 40 and mating slots 42 for a total rotation, one relative to the other, approximately one half of the backlash described previously. When the pin 34 engages the saddle-shaped depressions 32 on either driver, the force of the contact, by design of the saddles, will be angled outward from the plane of the respective driver and will overcome the component of the reaction force acting opposite created by the inclined edges on the mating teeth on the drivers 30 and couplers 22. For example, saddle angles ranging from 30 to 40 degrees are typically used and create outward axial forces that exceed the inward axial forces created by typical 20 to 25 degree inclines of the coupler and driver mating teeth that would otherwise work to separate the driver from the coupler. Using the foregoing parameters, consider first the vehicle at rest. Assume the two drivers 30 each engage with the respective coupler 22, and for specificity in the starting condition, that the pin 34 is centered in the saddle-shaped depressions 32 in the drivers 30. With the vehicle in gear and engine driving, the pin 34 begins to rotate about the axis of the axle, through the backlash present and compressing against springs 36 to contact the edges of the saddle-shaped depressions 32 in the drivers, and then on further rotation, to force the drivers and couplers, and thus the axles, into rotation. Because the contact angle between the pin 34 and the saddle-shaped depressions 32 exceeds the angle of the edge of the teeth on the couplers and drivers, the force between the pin and the drivers forcing the same into contact against the couplers 22 will exceed the force between the inclined edges of the teeth on the drivers 30 and couplers 22 otherwise tending to force the drivers back toward pin 34, so that the drivers and couplers will remain in positive engagement, regardless of the torque applied to the differential.
If the vehicle now proceeds to drive around a curve, the wheel on the outside of the curve, and thus the coupler 22 associated with that wheel, will tend to rotate faster than the coupler associated with the inside wheel. Assuming power is still being applied, this causes the driver associated with the outside wheel to begin "gaining" with respect to pinion pin 34, the driver rotating forward to a position wherein the saddle-shaped depressions 32 thereon are no longer in contact with pin 34. At this point, pins 40 and mating slots 42 prevent the further relative rotation of the two drivers but allow coaxial translation. Further gaining of the outside wheel continues to rotate the outside coupler at a speed higher than the other differential components. Now, however, the teeth on the driver associated with the outside wheel are free to climb the inclined planes of the teeth on the driver and coupler, with the driver moving toward the pin 34 against the resistance of the associated springs 36 to allow the teeth of the respective driver to slide over the teeth of the respective coupler, repeatedly as required so long as the difference in wheel rotation speeds exist.
If, when in a curve, the vehicle engine is throttled back to coast and the engine is used as a braking or vehicle slowing device, the same basic interaction of parts described above will occur substantially in reverse, now however with the driver and coupler associated with the outer wheel of the curve being engaged, and the driver associated with the inner wheel of the curve climbing over the teeth on the associated coupler as required to allow the inner wheel on the curve to turn slower than the outer wheel. Similarly, in backing around a curve such as backing out of a parking place, the inner wheel will be the drive wheel, as in powering forward, whereas use of the engine to retard the motion of the vehicle when backing will engage the wheel on the outer side of the turn. However in any event, when power is applied while turning to the point that traction is lost by the drive (inside) wheel, pin 34 will catch up to and forcibly engage the appropriate side of the saddle-shaped depression 32 on the outside wheel driver 30, forcing both drivers into engagement with their associated couplers to force rotation of both axles in unison.
The foregoing locking differential and another manufactured by Vehicular Technologies called "Lock Right" perform well, allowing the drive wheels to rotate independently under normal conditions, but causing the wheels to rotate in unison when either wheel loses traction and begins to slip. The Lock Right design differs from the aforementioned design in that it has no springs located in the saddle to dampen the backlash, rather springs are located between the drivers and thus work directly to force the driver teeth into mesh with those on the adjacent coupler. However, these differential designs contain a few particular operating characteristics that may require the vehicle operator to become accustomed to.
In particular, when one wheel begins turning faster than the other, such as when turning into a parking space, one driver will be climbing the teeth on the associated coupler and sliding thereover. When the teeth of the driver again align with the spaces between teeth on the coupler, the driver will fall into engagement with the coupler and shortly thereafter climb the sides of the teeth and again disengage. This makes an audible noise, resulting in a "click, click, click" type sound heard from outside the vehicle. In louder vehicles, such as a high performance vehicle, particularly for one technically versed to understand the source of the sound, the sound is of little consequence. However, to the driver of a more typical, quieter car, the sound can be a distraction, and could be misinterpreted as a mechanical fault or impending mechanical failure. Secondly, a phenomenon called "cycling" can be induced in manual transmission equipped vehicles. Automatic transmissions do not exhibit the condition because the torque converter always maintains a measure of bias load between the engine and drive axle. With manual transmissions, this event occurs when turning at slow speeds with the clutch pedal depressed, such as when turning into a parking space, temporarily decoupling the transmission from the engine and therefore removing any bias load present on the engaged driver and coupler. When the disengaged driver and coupler teeth pass by each other, they briefly reengage, enabling a load to be placed on the differential and axle components. The components between the differential and the wheel then act like an undamped mechanical spring and release the energy by temporarily accelerating the differential, drive shaft and transmission components. The inertia of these components carries the differential pin against the driver saddle, causing the opposite side driver and coupler to lock and continue to process. The continual wind-up and release will build and eventually become sufficient to "rock" the vehicle driveline and require the transmission be put in neutral or the vehicle stopped in order to cease the cycling. Needless to say, this is highly undesirable and would only be acceptable to the very most forgiving of owners. However all drivers whose vehicles represent a standard "as-delivered" condition would appreciate the increased traction a locking differential provides in situations where dry pavement type traction is not available. It would therefore be desirable to provide a differential with substantially the same simplicity as the differentials just described, but which is quiet and smooth in its operation, so as to neither be heard by nor concern the average driver of a vehicle equipped with the locking differential.
Referring again to FIG. 1, recessed areas 86, which are in the face of the drivers, may be seen. These recessed areas are to provide a space through which the C clips 24 may be inserted once the drivers and couplers have been assembled into the differential case and the axles put adjacent their final axial position. However the recessed areas 86 interrupt the continuity of the distribution of the teeth on the drivers, creating a lack of symmetry therein. This, in turn, creates a cocking force between the drivers and respective couplers which, particularly for less than full tooth engagement, results in the plane of the drivers and couplers not remaining accurately parallel. Consequently, the load on the teeth of the drivers and couplers is not equally distributed among the teeth. Further, for any two engaging teeth, the load on the area of engagement of the teeth is not equally distributed over the engaging area. The net result is faster tooth wear than necessary, and reduced resistance to abuse because of load concentrations. Also, recessed areas 86 have to be located at an axial position where the C clip 24 may be slid onto the C clip groove in the axle. This may not be the final position of the axle, but it must be in the insertable range of the axle into the differential assembly. This predetermines limits for the axial position of the slot. in turn constrains the relative thicknesses of the drivers and couplers, preventing the optimization thereof to equalize the strength of the drivers and couplers.
Another prior art differential is the Detroit Locker manufactured by Tractech, a Titan Wheel Company. In this "locking" type of differential, a cammed spider ring member takes the place of the differential pin and contains negative angle teeth that transfer torque to negative angle teeth on a driven clutch member which is able to slide axially on the splined barrel of the adjacent side gear. A single coil spring holds each driven cam against the spider. The side gear interfaces with the axle through another splined connection. While the driven clutch members separate from the central spider during turns, it is noteworthy to mention that a cam centrally located within the spider forces the driven clutch of the non-driving wheel away from the spider to a point of sufficient separation so that the separation device coupled to the driven clutch can be positioned to maintain the teeth of the driven clutch with slight engagement with the teeth of the spider until relative rotation between the driven clutch and spider again reverses direction. Because the teeth of the driven clutch slightly engage the teeth of the spider before passing over respective corner radii, there is slight sound when turning into a parking place.
While the Detroit Locker mechanism is functional, it is somewhat complex and in general cannot be assembled into pre-existing single piece differential cases because of access limitations. Accordingly, for the vast majority of vehicles, the original equipment differential case must be removed and a new differential case inserted with the locker assembly therein. This has the disadvantages of the extra cost of the new differential case, as well as requiring relocating the ring gear from the previous differential case to the Detroit Locker differential case, and again setting up the ring gear and drive shaft pinion gear for proper backlash and alignment for proper operation. In contrast, the positive acting differential developed by Vehicular Technologies herein described can be assembled into original equipment differential cases most often without removal of the differential from the vehicle, avoiding the expense of a new differential case and the time and skill required to again set up the spacing between the ring and pinion gearset.